The goal of the proposed research is to study the differentiative events which occur during the transition of muscle progenitor cells, myoblasts, into functional muscle cells and the manner in which this transition is coupled to the cell division cycle of the progenitor cell. Culture conditions have been devised in which fusion-blocked myoblasts, synchronized in M complete one round of synchronous division, blocking in the second G1 period. In this second, protracted G1, virtually the entire population (ca 90%) initiates myosin synthesis, in good synchrony beginning 8 hours after completion of the previous mitosis. This system will be employed to map the initiation of synthesis of additional muscle specific proteins and their non-specific homologs as a function of cell cycle phase since the differentiative transition involves both the expression of a set of muscle specific gene products and the repression of gene activities expressed in the progenitor cell (and more ubiquitously distributed). Our recent evidence suggests that the rate of synthesis of muscle-specific CPK subunits increases after fusion but that there is also a progressive decrease in the rate of synthesis of the subunits of the non-specific CPK homolog reaching undetectable levels 8 hours after the switch to fusion-promoting medium. Experiments are planned to measure rates of degradation of the muscle specific and non-specific CPK isoenzymes and to account for the rates of accumulation of active enzyme. Plans are outlined to isolate temperature sensitive mutants of established myoblast lines which block in G1 to determine whether or not any portion of the pre-replicative program of G1 is required for differentiation and complementation tests are described.